The present invention relates to a production method of a porous medium and specifically to a production method of a photographic ink jet recording medium.
In recent years, ink jet recording materials have been increasingly improved in image quality which is more and more approaching conventional photographic quality. Specifically, ink jet recording sheets have been improved to meet the need, in order to achieve image quality equivalent to conventional photographic quality employing ink jet recording. As a result, a porous type recording sheet, which comprises a highly smoothed support having thereon an ink receptive layer comprised of pigments and hydrophilic polymers, has become one of the ink jet recording sheets which result in conventional photographic quality due to high glossiness, bright color formation, and excellent ink absorbability as well as excellent ink drying properties. When a non-water absorptive support is employed, cockling (also called wrinkling), which is common for water absorptive supports, does not occur after printing, whereby it is possible to maintain the highly smoothed surface, resulting in higher quality prints. Further, when water-soluble dye ink is employed, it is possible to produce color prints with image quality equivalent to conventional photography which results in clear high-definition and uniform surface glossiness.
Of these ink jet recording materials, particularly a porous medium of a microscopic pore diameter of 20–50 nm has increasingly been used as an ink jet recording sheet from the latter half of the 1990s and has established its own industrial position. Characteristics required for a porous medium as a photographic ink jet recording sheet include: (1) a uniform surface or occasionally a highly glossy surface, (2) rapid absorption of deposited ink droplets, and (3) sufficient color formation. Widely known as ink jet recording sheets exhibiting such characteristics are those which are prepared by coating a liquid coating composition, comprising inorganic microparticles of a microscopic diameter, and hydrophilic binders (refer to, for example, Patent Document 1). During the preparation, it is commonly known that the coating weight of inorganic microparticles is to be at least 10 g/m2, and the weight ratio (F/B) of the inorganic microparticles (F) to the hydrophilic binders (F) is to be 2–20. However, the following are problems. The employed liquid coating composition is very delicate and coagulants tend to result depending on specific additives. The resulting porous layer is fragile, whereby cracking tends to occur due to rough handling during coating and drying, or after drying. These problems markedly occur when inorganic microparticles at an average diameter of the primary particles of less than or equal to 30 nm are specifically employed.
On the other hand, a method is known in which urea or its derivatives are employed as a plasticizer for binders in the porous ink receptive layer which constitutes an ink jet recording sheet. For example, a recording sheet is proposed which comprises a substrate having thereon a porous ink receptive layer comprising at least one compound selected from the group consisting of urea derivatives, semicarbazide derivatives, carbohydrazide derivatives, and hydrazine derivatives and exhibits excellent dye fixability (refer, for example, to Patent Document 2). However, the above patent document refers to neither minimization of cracking during drying due to the presence of urea derivatives nor to desired effects for ink absorbability.
As one measure to overcome the above drawback, it is desired to form a porous layer while avoiding the use of undesirably affecting additives as much as possible, so that generation of coagulants is avoided, while maintaining the stability of the liquid coating composition. However, some additives are essential to realize the various characteristics required for the porous media. Consequently, many of these additives are occasionally incorporated into a porous medium, separately coated after forming the porous layer. In the case of long-run coating, in view of production cost, it is advantageous that after forming a porous layer, for example, the second coating is carried out prior to winding the precedent coating into a roll. Proposed as one of the examples is a method in which after the falling-rate drying zone, a solution comprising functional additives is applied onto the porous layer (refer, for example, to Patent Document 3).
However, in the method in which formation of the porous layer and the addition of functional compounds are separately carried out, the potential of generation of coating problems increases during the second coating, resulting in lowering the resulting yield. Consequently, it is desired that conditions which enable stable coating are selected as much as possible and the employed liquid coating composition exhibits characteristics which minimize such problems. Specifically, in the second coating, is a process in which the liquid coating composition comprising functional additives penetrates into the porous layer formed by the first coating. The inventors of the present invention made verification based on each of the proposed methods described above. As a result, it was discovered that characteristics of the porous layer formed in the first coating greatly affect final coating quality.
(Patent Document 1):
Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 11-348409 (claims)
(Patent Document 2):
JP-A No. 2001-10211 (Paragraph No. 0020)
(Patent Document 3):
JP-A No. 2002-3317456 (claims)